The invention relates to a process for producing nitrogen by low-temperature fractionation of air in a rectification system which has a high-pressure column and a low-pressure column, in which process charge air is introduced into the pressure column, an oxygen-containing liquid fraction is removed from the high-pressure column and fed into the low-pressure column, gaseous nitrogen is extracted from the low-pressure column above a mass transfer section which has at least one theoretical or practical plate, and is at least partially condensed in a top condenser by indirect heat exchange with a refrigerant, and high-purity nitrogen is removed from the low-pressure column below the mass transfer section and is obtained as nitrogen product.
Processes of this type and corresponding apparatus are known from EP 948 730 B1 and EP 955 509 A1. In these cases, nitrogen can be obtained under elevated pressure in the low-pressure column. If blocking plates (xe2x80x9cmass transfer section which has at least one theoretical or practical platexe2x80x9d) are built into the upper region of the low-pressure column, the nitrogen product can be of particularly high purity, and in particular can have a very low level of highly volatile impurities.
An object of the invention is to provide a process of the type described above and a corresponding apparatus which are particularly economically advantageous.
Upon further study of the specification and appended claims, further objects and advantages of this invention will become apparent to those skilled in the art.
These objects are achieved by a process which has refrigeration-supply system, in which a refrigeration fluid flows and at least part of the refrigeration fluid from the refrigeration-supply system is introduced into the low-pressure column above the mass transfer section. As an alternative, or in addition, within the context of the invention, it is also possible for the refrigeration fluid from the refrigeration-supply system to be introduced into the upper region of the high-pressure column.
In the invention, the refrigeration fluid may be formed by a readily available medium which is introduced into the low-pressure column and is in this way involved in the mass transfer in the low-pressure column, without the purity of the high-purity nitrogen product being impaired and without the operating pressure of the low-pressure column having to be matched to the requirements of the refrigeration-supply system. For example, if the refrigeration fluid is introduced into the low-pressure column the nitrogen content of the refrigeration fluid can be, e.g., at least 95 vol %, preferably at least 99 vol %. (An example of a suitable refrigeration fluid is nitrogen which still contains highly volatile impurities.) By contrast, in the refrigeration-supply system of the known processes, a residual fraction, for example from the evaporation space of the top condenser, is extracted, expanded in a work-performing manner to approximately atmospheric pressure and removed from the process. In this case, the minimum operating pressure of the top condenser and therefore that of the low-pressure column is determined solely by the refrigeration-supply system. This drawback is avoided in the invention without the product""s purity being reduced.
In the invention, the refrigeration fluid is preferably fed in at the top of the low-pressure column.
In a first variant of the invention, the refrigeration fluid is removed from the high-pressure column, expanded in a work-performing manner in the refrigeration-supply system and introduced into the low-pressure column.
The work-performing expansion of a fluid, in particular a gas, from high-pressure-column pressure to low-pressure-column pressure allows particularly expedient generation of process refrigeration. In this way, the insulation and exchange losses can be compensated for and if appropriate small quantities of product can be liquefied. Upstream of its work-performing expansion, the refrigeration fluid is preferably heated in indirect heat exchange against process streams which are to be cooled.
The mass transfer section is formed by one or more rectification plates (known as barrier plates)xe2x80x94for this purpose, the information is given in xe2x80x9cpracticalxe2x80x9d plate numberxe2x80x94or by a short packing section (xe2x80x9ctheoreticalxe2x80x9d plate number). The number of barrier plates or theoretical plates is, for example, 1 to 10, preferable 2 to 3. As a result of nitrogen product being extracted below these barrier plates, the nitrogen product has a very low level of the highly volatile impurities which remain in the top of the low-pressure column and, from there, are extracted with a less pure nitrogen stream.
The refrigeration fluid generally contains constituents which are more volatile than nitrogen. However, on account of the feed taking place above the mass transfer section described, these constituents do not pass into the nitrogen product which is extracted further down.
It is expedient if the refrigeration fluid is removed from the upper region of the high-pressure column. It is formed, by way of example, by a nitrogen-rich gas fraction from the high-pressure column, in particular by the top gas of this column.
According to a second variant of the invention, the refrigeration fluid is formed by a cryogenic liquid which has been produced outside the rectification system.
As a result of external fluid being fed in as refrigeration source (liquid assist), the process has a particularly high flexibility. By the way of example, machines for producing refrigeration, such as for example expansion turbines, can be completely or partially dispensed with. The cryogenic liquid may be formed, for example, by liquid nitrogen which originates from another air fractionation installation; alternatively, any other mixture of air components can be used. The external liquid may either be supplied via a pipeline or be removed from a storage vessel. It is fed in at that point which corresponds to the composition of the external liquid. This may be the upper region of the high-pressure column or low-pressure column.
The cryogenic liquid may be partially or completely introduced into the low-pressure column, preferably at the top of this column. As an alternative or in addition, the cryogenic liquid may be at least partially introduced into the upper region of the high-pressure column.
Refrigerant for the top condenser is preferably removed from the lower region of the low-pressure column, and all the oxygen-enriched product of the pressure column is fed into the low-pressure column. In the present context, the term xe2x80x9coxygen-enrichedxe2x80x9d is understood as meaning any fraction whose oxygen content is greater than that of air.
The nitrogen product may be extracted in gas form from the low-pressure column. Alternatively, it is removed in liquid form from the low-pressure column and evaporated in indirect heat exchange with refrigeration fluid which has been expanded in a work-performing manner. A combination of these two process steps is also possible.
In addition, the invention relates to an apparatus for producing nitrogen by low-temperature fractionation of air in a rectification system comprising:
a high pressure column (4) and a low-pressure column (5);
a feed line (1, 3) for introducing air into the high-pressure column (4);
a line (11) for introducing an oxygen-containing liquid fraction from the high-pressure column (4) into the low-pressure column (5);
a top condenser (17), the liquefaction side of which is connected to a region of the low-pressure column (5) above a mass transfer section (25), the mass transfer section having at least one theoretical or practical plate;
a nitrogen product line (26, 27, 30) for removing high-purity nitrogen, which is connected to the low-pressure column (5) below the mass transfer section (25); and
a refrigeration-supply system, which has a refrigeration-fluid line (31, 32, 34; 38) connected to the low-pressure column (5) above the mass transfer section (25).
In accordance with a further aspect of the apparatus, the refrigeration-supply system has an expansion machine (33), the inlet of which is connected (11, 32) to the high-pressure column (4) and the outlet of which is connected (34) to the refrigeration-fluid line (31, 32, 34; 38).
According to a further aspect of the apparatus, the refrigeration-fluid line is a liquid line (38) for feeding into the refrigeration-supply system a cryogenic liquid produced outside the rectification system.
The invention and further details of the invention are explained in more detail; below with reference to the two exemplary embodimentsxe2x80x94one for each of the two variants of the inventionxe2x80x94illustrated in the drawing.